1. Field of the Invention
The present invention pertains to seismic data processing and more particularly to pre-processing seismic data in which data generated by a vibrating source is received and prepared for high resolution or high fidelity data processing.
2. Related Prior Art
It is conventional practice to use a vibratory source to apply a force to the ground and measure the subsequent motion caused by the application of this force at various receiver locations. By controlling the duration and frequency of the force a broad band signal with sufficient energy is achieved. By using the receiver motions and assumed force application a seismogram is constructed (typically by correlation with an estimate of the applied force) from which properties of the impedance function of the earth can be calculated.
The main deficiency of conventional practice is that an estimate of the actual applied force is used to create the seismogram. Much work has been done in order to improve the quality of feedback signals and the operation of feedback loops and hydraulic valves. However, harmonics, device flexure and variable ground coupling remain as unknowns in the system.
In conventional processing, data that is generated by a vibratory source is correlated with a reference sweep. A reference sweep signal is an ideal signal which the vibrator is told to put into the ground, which is often quite different from the actual signal which is generated. The typical estimate for the applied force is the mass weighted sum of the acceleration of the baseplate used in the vibrating source and the acceleration of the reaction mass used in the vibrator structure, called the ground force.
Traditionally, a reference sweep is created and fed into an actuator. The actuator vibrates and attempts to put a ground force identical to the reference sweep signal into the ground. Usually there are two accelerometers on the vibrator, one on the baseplate and one on the reaction mass used with the vibrator structure. Conventional techniques assume that the vibrator earth model has a base plate that is stiff, although it is known that there is a lot of flexing in the base plate. This can inject inaccuracies in processing methods since prior art methods, based on allowing the base plate to flex, are approximations.
The mass weighted sum of the two signals, one from the baseplate and one from its reaction mass, is used in a feedback loop to tell the actuator how close it is to the reference sweep. With this system it is assumed that the force injected into the ground is the same as the reference sweep. However, as stated previously, the actual signal is often very different from the reference sweep signal.
The force put into the ground can be viewed either in the time domain or in the frequency domain. Similarly, the impulse response of the earth can be viewed either in the time domain or the frequency domain. The time derivative of the force put into the ground is convolved with the impulse response of the earth in the time domain while the time derivative of the force is multiplied by the impulse response of the earth in the frequency domain. In its most basic form, a signal representing the derivative of the ground force convolved with the impulse response of the earth is detected by geophones or receivers located on the surface of the earth. It is detected after it has been reflected by an interface existing between two subsurface layers having different impedances. The detected signal is correlated with the reference sweep signal fed to the actuator. This correlation works fine to compress the force portion of the detected signal in a known way as long as the force put into the ground is the same as the reference sweep signal. Since it is rarely the same, an accurate estimate of the impulse response of the earth is seldom achieved.
Correlation in the frequency domain requires that the data be multiplied by the time reverse of whatever signal with which the correlation is being done. Since the reference is only an estimate of the actual ground force, the result is that an unknown is still in the data. In the case of correlation of the signal with the reference, the unknown does less damage to the result as long as the amplitude and phase errors of the reference signal are small, but it still adds error.